Past Seminars

High-n Ballooning Modes in the Nonlinear Stage of Sawtooth Crash

Yasuturo Nishimura: CIPS - University of Colorado at Boulder

A tokamak sawtooth crash phase has been studied numerically By starting from a concentric equilibrium, it has been shown that the evolution through an $m/n=1/1$ magnetic island induces secondary high-$ n$ ballooning, (Rayleigh-Taylor-like) instabilities. The magnetic island evolution gives rise to convection of the pressure inside the inversion radius and builds up a steep pressure gradient across the island separatrix, or the current sheet, and thereby triggers ballooning instabilities below the threshold for the axisymmetric equilibrium. Due to the onset of secondary ballooning modes, concomitant fine scale vortices and magnetic stochasticity are generated. These effects produce flows across the current sheet, and thereby modify the $ m=1$ driven magnetic reconnection process. The resultant interaction of the high-$ n$ ballooning modes with the magnetic reconnection process is discussed.

Electrical and Optical Properties of Ice

Niyaz N. Khusnatdinov: Dartmouth College

Ice, being an important substance to our life, possesses unique physical properties which attract attention of many researchers. Some of those properties are poorly understood even today. For instance, formation of a liquid-like layer on the ice surface is mostly unexplained. Other properties like electrical and optical are better studied, though leaving many unsolved questions. In part slow progress is explained by experimental difficulties one encounters trying to work with ice. High vapor pressure, low atomic weight, hydrogen bonding, liquid-like layer and protonic electric conductivity make it very difficult and sometimes impossible effectively apply modern techniques and methods used to study other solids. From the standpoint of the electronic conduction theory, ice, with its wide band gap, E(gap)= 10.9 eV, must be an insulator. However, it exhibits significant electric conductivity ( ~10-7 (Ohm cm)-1 at T = - 10?C), which is protonic by nature. The electric conductivity is explained by the motion of so called defects of protonic subsystem: ions and Bjerrum defects. In total there are four different charge carriers. Practically all properties of ice including mechanical strongly depend on the state of protonic subsystem, concentration of charge carriers and their mutual proportions. The talk will describe the protonic photoconductivity of ice, the photo-plastic effect in ice, and methods used to investigate the nature of photo charge carriers.

Surface Waves and Magnetic Reconnection in Space Plasmas

Chanchal Uberoi: Indian Institute of Science

A time -dependent theoretical model of surface wave induced magnetic reconnection ( SWIMR ) based on the mathematical analysis of resonant absorption of Alfven waves near a neutral point is briefly reviewed. Keeping in view recent critical rewiews of vortex- induced magnetic reonnection (VIMR ) it is argued that SWIMR may be a more plausible mechanism to explain the FTEs at the magnetopause. It is further suggested that SWIMR may be responsible for the recently observed plasmoid ejectons from the microflares in the solar corona.

Hong Qin: Princeton University

A linear gyrokinetic system for low frequency electromagnetic modes is developed. A wide range of modes in inhomogeneous plasmas, such as the kink modes, the compressional and shear Alfv\'en modes, and the drift modes, can be recovered from this system. The inclusion of most of the interesting physical factors into a single framework enables us to look at many familiar modes simultaneously and thus to study the modifications of and the interactions between them in a systematic way. Especially, we are able to investigate selfconsistently the kinetic magnetohydrodynamics (MHD) phenomena entirely from the kinetic side. Phase space Lagrangian Lie perturbation methods and a newly developed computer algebra package for vector analysis in general coordinate system are utilized in the analytical derivation. A two dimensional finite element code has been developed and tested.

Simulation of Wave-particle Resonant Interactions with New delta-f Method

Yang Chen: PPPL

I will briefly review the physics of the saturation of kinetic instabilities due to wave trapping, and the role of collisions in wave-particle resonant interaction. I will provide a general introduction to the delta-f method, then discuss some of the difficulties in extending delta-f method to cases with collisions, and show how the introduction of a convenient tool (the marker distribution in extended phase space) overcomes such difficulties. I will then try to apply the new method to the simulation of a recent TFTR experiment and discuss the result.

Slow Dynamos in Hexagons and Fast Dynamos in Cubes and Their Possible Astrophysical Relevance

David Galloway: University of Sydney

I will discuss numerical solutions of two distinct problems in dynamo theory. The first calculation is kinematic, and investigates the generation of magnetic fields by a horizontally periodic layer of identical hexagons. This flow acts as a dynamo for sufficiently high fluid conductivity, even though it has zero helicity and is purely poloidal. It is integrable, and must therefore be what is termed a slow dynamo (the distinction between fast and slow dynamos will be explained). The second calculation looks at field generation by the chaotic family of ABC flows. These are periodic in all three space directions. Kinematic calculations will be reviewed to illustrate the evidence that these flows yield fast dynamos. New results will be presented which feature the back reaction of the Lorentz force. These demonstrate that dynamos with very strong fields are indeed possible, though the examples studied have zero mean field. (The issue of whether or not dynamos can generate fields of the strengths observed in astrophysics is currently contentious.)

Buoyant, Interacting Magnetic Flux Tubes in the Solar Convection Zone

Yuhong Fan: JILA

Independent observational and theoretical developments have led to the current consensus that the solar dynamo mechanism responsible for the solar cycle is operating in the thin overshoot layer beneath the base of the convection zone. Therefore studying the physical processes involved in the transport of toroidal magnetic flux across the solar convection zone and its emergence as active regions becomes an important part of understanding solar magnetic activity. Although investigations based on a one-dimensional thin flux tube model have produced interesting results which explain the origin of some basic observed features of solar active regions, it is a highly simplified picture with severe limitations. Recently, multi-dimensional MHD simulations have been carried out to investigate aspects of the dynamics of emerging magnetic flux tubes that cannot be addressed by the 1-D thin tube model. In this talk we present the results of two-dimensional simulations of the buoyant rise and mutual hydrodynamic interactions of twisted, horizontal flux tubes in a stratified layer representing the solar convection zone.

The Change of the Elemental Abundances on the Sun from the Photosphere to the Solar Wind

Hardi Peter: Max Planck Institution

About 50 years ago it became clear, that the the Sun is loosing mass (and angular momentum) due to a continuous outflow of material: the Solar Wind. The interaction of the Solar Wind with the Earth's magnetosphere sometimes can have an impact on everyday life; the northern lights are the most impressive result of this interaction. To understand the origin and the acceleration of the Solar Wind it is of importance to understand the processes in its source region, namely the solar chromosphere and corona. The physical parameters, e.g. temperature or density, are determined with the help of spectroscopy. Because the relative elemental abundances are crucial for the line ratios, one has to understand the change of the abundances throughout the chromosphere and corona. The abundances may also have an influence on the heating of the solar corona and they can be used as a tool to map the Solar Wind back to the solar photosphere, to locate its source region. The observations show, that elements with a first ionization potential (FIP) below 10eV are enriched compared to those with a higher FIP typically by a factor of 4. An ionization-diffusion mechanism is proposed to understand these observations in a large variety of phenomena, as e.g. slow and fast wind or polar plumes.

General Properties of Transport in Plasmas: An Overview of Onsager Symmetry

Igor Manuilskiy: PPPL

Onsager symmetry (symmetry of the off-diagonal coefficients of the transport matrix) is one the most fundamental properties of any macroscopic system in equilibrium (indeed, it's a classical thermodynamics textbook topic). Nevertheless it has been an issue of a controversy in plasma community. The core of the argument is the question of symmetry's applicability - originally derived for equilibria - to turbulent conditions. I will review the topic, from the classic equilibrium theory to recent results which extend the validity of Onsager symmetry to the steady-state non-equilibrium (turbulent) situations. The foundation of the symmetry in reversible microscopic dynamics will be discussed. A covariant formulation of the problem will be considered. Special attention will be paid to the cases of Onsager symmetry in tokamak plasma (classical, neoclassical and turbulent).

Mikhail Medvedev: UCSD

Structure formation in the interplanetary magnetic field is a common feature of collisionless space plasma turbulence. It often occurs in the form of discontinuities, shock-trains, and nonlinear waves. Nonlinear dynamics of high-amplitude (compressible) Alfven wave trains subject to collisionless (Landau) dissipation is investigated in both "single wave" and turbulent regimes. In beta=1 plasmas (typical for the solar wind), where the traditional DNLS-based approach fails, the effect of resonant particle-wave interactions is shown to lead to the formation of various types of Alfvenic discontinuities (e.g., arc- and S-polarised rotational discontinuities), which are commonly observed in the solar wind in spacecraft observations. In the turbulent regime, two different regimes (or phases) of (compressible) Alfvenic turbulence are predicted. The inflience of other kinetic effects (e.g., collisions, gyro-kinetic effects, trapping of particles by a wave) on the wave dynamics is also investigated.

On the Io Torus Electron Velocity Distribution

Doyle Hall: LASP

Understanding the velocity distribution of the electrons in Jupiter's Io Plasma Torus is an important consideration when studying the radiation emitted from the plasma and the flow of energy through the system. Torus electrons are known to be much cooler than torus ions (by a factor of roughly 10) because they are efficiently cooled by e-impact radiative collisions with sulfur and oxygen ions. The distribution fe(v) is also known to possess a high-energy tail that is thought to be produced (in part) by interactions with plasma waves. Results from a new kinetic model that includes e-e and e-i coulomb collisions, e-i radiative collisions, as well as plasma wave electron acceleration will be presented, with particular emphasis on understanding the overall energy budget of the Io torus plasma.

New Kinetic Simulation Model for Wave-wave and Wave-particle Interactions in the Auroral Ionosphere

Kevin Y. Sanbonmatsu: CIPS & APS - University of Colorado at Boulder

Spacecraft frequently measure intense bursts of Langmuir waves in conjunction with field aligned electrons in the auroral ionosphere. A numerical simulation model which includes both wave-wave and wave-particle effects has been constructed. This model is based on differential equations, and allows us to examine the evolution of the extremely weak precipitating electron beams observed in the auroral ionosphere. These beams are often difficult to study using conventional particle-in-cell techniques, due to the relatively high noise levels of these simulations. Our model couples the 1-D quasilinear diffusion equation (which evolves the electrons) to the 2-D magnetized Zakharov equations (which evolve the wave spectrum), evolving the electron distribution function and Langmuir wave spectrum self-consistently. Previous self-consistent simulations have been 1-D. Previous 2-D studies have not been self-consistent, assuming either a fixed electron distribution or a fixed wave spectrum. Preliminary results of the new quasilinear-Zakharov model show that wave-wave and wave-particle effects act on similar timescales. Furthermore, forward scattering of Langmuir waves tends to heat the core of the electron distribution. This effect does not occur in 1-D.

Dusty Plasmas Experiments

Scott H. Robertson: CIPS

Several dusty plasma experiments in progress in the plasma laboratory will be described. 1) The charge on particles dropped through a low density plasma has been measured and compared with theory. The particles, 20 - 100 microns in diameter, are smaller than the Debye length, and it is confirmed that their charging is described by the orbit- limited theory of Langmuir probes when secondary emission is properly included. These measurements have recently been extended to a granular sample of lunar regolith returned by Apollo 17 astronauts. 2) An instrument developed for detecting charged dust particles is being modified to be flown on a rocket. The rocket instrument will detect charged atmospheric aerosols in the polar mesosphere and is expected to return data on the abundance and charge state of particles in noctilucent clouds. Launch is scheduled for August of 1998. 3) Tabletop projects involving undergraduates include a) observations of dust trapped for 6 or more hours in orbit about a sphere in vacuum with applications in celestial mechanics and b) trapping of clouds of charged droplets in a Paul trap and observations of modes of oscillation relevant to non-neutral plasmas.

Polar Auroras: A Manifestation of Instability Within a Thin Current Sheet in the Magnetotai

Tuija Pulkkinen: LASP

The plasma environment in the comet-like tail in the Earth's nightside forms an interesting plasma laboratory for studies of tenuous (density ~1/cc, temperature ~few keV) plasmas. Various observational and theoretical studies have recently suggested that a key element in the growth of large-scale instabilities is the formation of a thin (of the order of ion gyroradius) current sheet within the plasma sheet. The dynamic processes in the magnetotail are manifested in the polar ionospheres as bright auroral displays created by particle precipitation into the upper atmosphere. Thus, the auroral observations provide a projected image of the large-scale magnetospheric processes, which otherwise are difficult to capture due to the vast size of the system. This talk addresses the plasma environment in the nightside magnetotail during magnetospheric substorms. Model results of the thin current sheet formation prior to the auroral breakup are presented, and the stability conditions for the tail are discussed. The auroral observations during substorms are related to the tail processes by magnetic field-aligned mappings.

Modeling Large-scale Neutral Plasma Interactions in the Thermosphere and Ionosphere

R.G. Roble: NCAR-HAO

The mesosphere, thermosphere and ionosphere are highly complex regions of the Earth's atmosphere, with interacting dynamical, chemical, radiative, and electrical variations that are strongly coupled with the magnetosphere and lower atmosphere. To understand how these coupled systems interact to produce the great variability observed in the upper atmosphere is one of the major problems in space physics today. One tool that has been developed to study this region of the atmosphere is the NCAR Thermosphere - Ionosphere - Mesosphere - Electrodynamics General Circulation Model (TIME-GCM) that incorporates many of the aeronomic processes necessary to simulate the structure and dynamics of the region and determine its response to solar and auroral variability. The TIME-GCM has been used to study large-scale thermosphere and ionosphere dynamics and interacting electrodynamics for a variety. of geophysical conditions. The results of simulations with the TIME-GCM for both quiet and distrubed geomagnetic with be discussed.

Dusty Plasmas in Space

: LASP

Comets, planetary rings, asteroids, the Moon are all examples where dust grains and plasmas coexist. Dust particles collect electrostatic charges and act as sinks or sources for the density, momentum and energy of their plasma environment. I will briefly discuss the most important processes and show examples where dusty plasma theories were successful in explaining the observed spatial distribution of small dust particles.

Scattering of Spiral Density Waves to Lower Arm Number

Nathan Mattor: LLNL

Spiral density waves arise frequently in astrophysical disks, such as galaxies and protostellar nebulae. We have discovered a new process undergone by these waves, in which they couple and scatter to lower azimuthal mode number. The upshot is that wave energy undergoes downscattering, in which an initial perturbation evolves to an ever more symmetric state. Downscattering offers explanations for several diverse phenomena, such as the prevalence of galaxies with low numbers of arms, the formation of binary stars, and angular momentum transport in star formation. The scattering process is a fluid analogue to nonlinear Landau damping, in which two linear modes exchange energy, via the resonance of their beat mode. This process has been observed in recent laboratory experiments with diocotron modes in a pure electron plasma, which follow similar (not identical) dynamics. In this work, we calculate the scattering rate for a thin fluid disk model using a weak turbulence expansion, and producing an analytical formula for the scattering rate. This formula clearly shows the tendency to scatter to lower azimuthal mode number. This direction of scattering results from the tendency of disks to minimize free energy via inward mass and outward angular momentum transport. We apply the predicted rates to a model galaxy, showing scattering to be a robust mechanism, comparable in magnitude to typical linear growth rates.

Io's Interaction With Jupiter's Magnetosphere

Frank Crary: APS

The interaction between Jupiter's magnetosphere and its moon, Io, causes strong field aligned currents, and produces intense decametric radio emissions and auroral activity. After the discovery of these radio emissions in 1955, this interaction was believed to involve a steady current flow across Io, induced by Io's orbital motion across Jupiter's magnetic field lines. These currents would flow along the field lines and close through Jupiter's ionosphere. After the Voyager spacecraft observed a dense plasma torus around Io, in 1979, this model seemed unlikely: Based on propagation times and flow velocities, it seemed that the system would not settle into equilibrium. Instead, the interaction would take the form of a Alfven wave propagating away from Io and towards Jupiter. Now, with the new results of the Galileo spacecraft, our understanding of the Io interaction may change again. In this talk, I will describe the previous theories of the Io interaction, and the the time scale arguments that determine whether the interaction is one of steady currents or an Alfvenic disturbance. I will then discuss my own work on the nature of the interaction and how it couples to the high-latitude phenomena. After summarizing the Galileo results, I will present a new model for the interaction, motivated by the Galileo data, and the implications for Jovian radio emissions and aurora.

Magnetic Fields in Galaxies and Beyond

Ellen Zweibel: JILA & APS

The importance of magnetic fields in determining the structure, energy balance, and star formation history of galaxies is universally accepted, but the origin and evolution of galactic magnetic fields is still uncertain. I will discuss our current state of knowledge and some unsolved problems.